Practical Ship Hydrodynamics

80 Practical Ship Hydrodynamics
A curve for the parameter KT/J2 as function of J is introduced in the openwater
diagram for the full-scale ship. The design point is defined by:
�
KT
J 2
�
Ts
cTs
D
Ð
⊲1 t⊳⊲1 ws⊳ 2
s
s Ð D 2 s Ð V2 As
D Ss
2D 2 s
The curve for KT/J 2 can then be used to determine the corresponding Js.
This in turn determines the torque coefficient of the propeller behind the
ship KQs D f⊲Js⊳ and the open-water propeller efficiency 0s D f⊲Js⊳. The
propeller rpm of the full-scale propeller is then:
ns D ⊲1 ws⊳ Ð Vs
JsDs
The propeller torque in full scale is then:
Qs D KQs
R
s Ð n 2 s Ð D2 s
The propeller thrust of the full-scale ship is:
�
Ts D Ð J 2 s Ð s Ð n 2 s Ð D4 s
� KT
J 2
The delivered power is then:
PDs D Qs Ð 2 Ð ns
s
The total propulsion efficiency is then:
Ds D 0 Ð R Ð Hs
3.3 Additional resistance under service conditions
The model test conditions differ in certain important points from trial and
service conditions for the real ship. These include effects of
ž Appendages
ž Shallow water
ž Wind
ž Roughness
ž Seaway
Empirical corrections (based on physically more or less correct assumptions)
are then used to estimate these effects and to correlate measured values from
one state (model or trial) to another (service). The individual additional resistance
components will be briefly discussed in the following.
ž Appendages
Model tests can be performed with geometrically properly scaled
appendages. However, the flow around appendages is predominantly
governed by viscous forces and would require Reynolds similarity.
Subsequently, the measured forces on the appendages for Froude similarity